1. Field of the Invention
The field of art to which the claimed invention pertains is the removal of sulfur oxide from a gas, particularly a flue gas.
2. Background Information
Due to the ever increasing concern about air pollution, great efforts have been expended in recent years toward the development of processes to reduce the pollutants introduced into the atmosphere from various industrial operations. One of the most onerous of these pollutants is sulfur dioxide which is present in the stacks of flue gases from various operations. In one such operation, the fluidized catalytic cracking (FCC) process, sulfur compounds contained in the hydrocarbon feedstock result in sulfur containing material to be deposited on the FCC catalyst along with the carbonaceous material and thereby cause the generation of sulfur dioxide in the FCC regeneration section when the sulfur is burned off the catalyst along with the carbon deposits. This sulfur dioxide becomes a part of the regenerator flue gas and thus a pollutant when the flue gas eventually finds its way into the atmosphere.
There are many methods known to the art for removal of sulfur dioxide from stack or flue gases. There is, for example, the wet scrubbing process in which the sulfur dioxide reacts with an appropriate reactant contained in an aqueous solution or slurry sprayed into the flue gas, the sulfur thereby being removed from the system as a compound contained in the liquid phase. In another process the flue gas is passed through a fixed solid bed containing a sulfur "acceptor" with which the sulfur dioxide reacts and on which the sulfur is retained in the sulfate form, thereby being removed from the flue gas.
The basic prior art process for removal of sulfur dioxide from flue gas highly pertinent to the present invention is that disclosed in U.S. Pat. No. 4,071,436 to Blanton, Jr., et al. This reference discloses alumina and/or magnesia particles used to react with sulfur dioxide in an FCC regenerator flue gas to form a solid compound. When reacted with hydrocarbons in a reducing environment, such as the reactor, the sulfur is released from the acceptor. Hydrogen-containing gas is stated to be a less preferred reducing medium. This reference further states that it is preferred that materials such as lithium not be present in the particulate solid used for removal of the sulfur dioxide, since they simply form a nonregenerable sulfur-containing solid.
Other references having similar teachings as Blanton, Jr., et al. but not as relevant or no more relevant to the present invention are U.S. Pat. Nos. 4,153,535 to Vasalos et al.; 4,153,534 to Vasalos; 4,204,945 to Flanders et al.; 4,243,556 to Blanton, Jr.; 4,252,635 to Blanton, Jr.; 4,300,997 to Meguerian et al. and 4,325,811 to Sorrentino. The last mentioned reference also teaches the use of a reducing zone, separate from the reactor and regenerator, in which the sulfur laden acceptor is relieved of sulfur by reduction with hydrogen or a hydrocarbon gas.
The present invention is based on the discovery of an acceptor of a particular composition which has unique capabilities with regard to the disposition of sulfur oxides in a flue gas.